A placode in an embryo is a thickening of the ectoderm that will later develop into specialized sensory organs; a fish scale that is placoid is shaped like a plate; a placoderm (Greek derma, skin) is a fossil fish of the Devonian period, which has the front part of its body encased in broad flat bony plates; a placodont (Greek odous, odont-, tooth) is a fossil reptile of the Triassic period, which has short flat grinding palatal teeth.